Linearly fused acenes, like for example pentacene derivatives, are a promising class of molecular semiconducting materials (J. E. Anthony, Angew. Chem. Int. Ed., 2008, 47, 452; J. E. Anthony, Chem. Rev., 2006, 106, 5028). When deposited as a thin film by vacuum deposition, pentacene was shown to have hole mobilities in excess of 1 cm2 V−1s−1 with very high current on/off ratios greater than 106 (S. F. Nelson, Y. Y. Lin, D. J. Gundlach, T. N. Jackson, Appl. Phys. Lett., 1998, 72, 1854). However, vacuum deposition is an expensive processing technique that is unsuitable for the fabrication of large-area films. Device fabrication by solution processing was made possible by the introduction of solubilising groups to the pentacene core, namely, trialkylsilylethynyl groups, and yielded mobilities of up to 0.4 cm2V−1s−1 (C. D. Sheraw, T. N. Jackson, D. L. Eaton, J. E. Anthony, Adv. Mater., 2003, 15, 2009). Since then, there have been reports of further substitutions of the pentacene core unit to improve its semiconducting performance in field-effect transistor (FET) devices.
Anthra[2,3-b:7,6-b]dithiophene, which is an iso-electronic structure of pentacene, has been proved to be another class of molecular semiconductors with improved performances in field-effect transistors (M. M. Payne, S. R. Parkin, J. E. Anthony, C.-C. Kuo and T. N. Jackson, J. Am. Chem. Soc., 2005, 127 (14), 4986; S. Subramanian, S. K. Park, S. R. Parkin, V. Podzorov, T. N. Jackson, J. E. Anthony, J. Am. Chem. Soc., 2008, 130 (9), 2706) and in organic photovoltaic cells (M. T. Lloyd, A. C. Mayer, S. Subramanian, D. A. Mourey, D. J. Herman, A. V. Bapat, J. E. Anthony, and G. G. Malliaras, J. Am. Chem. Soc., 2007, 129 (29), 9144). Silylethynylated anthra[2,3-b:7,6-b]dithiophenes and their use in OFET and OPV devices are also disclosed in WO 2008/107089 A1 and U.S. Pat. No. 7,385,221 B1.
In order to further improve the carrier mobilities of the acenes, linear elongation by fusing additional aromatic rings have been attempted. However, this approach has been proven to impart instability and reduce the solubility of the materials (M. M. Payne, S. R. Parkin and J. E. Anthony, J. Am. Chem. Soc., 2005, 127(22), 8028. B. Purushothaman, S. R. Parkin and J. E. Anthony, Org. Lett., 2010, 12 (9), 2060).
However, the OSC materials of prior art, and devices comprising them, which have been investigated so far, do still have several drawbacks, and their properties, especially the solubility, processibility, charge-carrier mobility, on/off ratio and stability still leave room for further improvement.
Therefore, there is still a need for OSC materials that show good electronic properties, especially high charge carrier mobility, and good processibilty, especially a high solubility in organic solvents. Moreover, for use in OFETs there is a need for OSC materials that allow improved charge injection into the semiconducting layer from the source-drain electrodes. In addition, for OFETs to be applied in OTFT driving backplanes for selected display technologies, there is a requirement for the OSC materials to exhibit improved thermal robustness in order for compatability with annealing processes used in manufacturing.
It was an aim of the present invention to provide compounds for use as organic semiconducting materials that do not have the drawbacks of prior art materials as described above, and do especially show good processibility, good solubility in organic solvents, high melting points and high charge carrier mobility. Another aim of the invention was to extend the pool of organic semiconducting materials available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
It was found that these aims can be achieved by providing compounds as claimed in the present invention. In particular, it has been found that substitution of the terminal positions of anthradithiophene with sterically unhindered herteroaryls will lead to co-planar conjugated structure, as proved by single crystal X-ray structural analysis. In contrast to the linearly fusing technique as suggested in prior art, this type of derivatisation does not affect the stability and solubility of the materials significantly. However, this type of molecules has not been reported in the literature so far.
It was also found that OFET devices comprising the new compounds as semiconductors show good mobility and on/off ratio values, and can easily be prepared using solution deposition fabrication methods and printing techniques.